Numerical Simulation And Spin–coating/Electrospray Fabrication Of Organic Solar Cells

With the expansion of requirement and consumption for energy, the research on clean, greenand low cost new energy sources is urgent. Organic solar cell (OSC) has merits of low cost,flexibility, portability and the potential to roll-to-roll production, which has been one of the mostpromising fields of new energy. To improve the efficiencies of OSC devices and promote theprocess of application, research interests on OSC mainly focuses on four aspects: developing activematerials with wide spectral band and high carrier mobility, optimizing device structure by theorysimulation, modificating electrodes to improve carrier transport and collection, as well asdeveloping fabrication methods with potential to make flexible and/or large area devices. Thisthesis mainly focuses on the theory simulation, cathode modification and electrospray fabricationmethod of OSC devices.By simulating the photo-electron conversion process of OSC devices, and combining thesimulation and experimental results, the mechanism of OSC devices can be understood deeply.Moreover, theory simulation can also optimize device structure and instruct the device experiments.A software system is designed according to optical and electrical models as well as numericalsimulation, which can be used to simulate the electromagnetic field distribution and powerconversion efficiency of OSC devices. The optimized thicknesses of the donor, acceptor, andcathode buffer layers are obtained from simulation. Combining simulation results with theexperimental results, the relationship between the characteristics and structure of planerheterojunction OSC based on CuPc/C60is analyzed and the structure of device is optimized. Thisoriginal analysis method and corresponding software can be applied to structure design andcharacteristic simulation of OSC devices with different materials and different active layerstructures. This is important for understanding mechanism and pretending experimental results ofOSC devices.Zn4O(AID)6is initiatively adopted as cathode buffer layer in the bulk heterojunction OSCdevice based on the spin coated P3HT:PCBM active layer. Experimental results show that thiscathode buffer layer can improve device efficiency up to70%. The effects of using Zn4O(AID)6ascathode buffer layer on performance of device are investigated by combining the experimental andsimulation results. The analysis results indicate that Zn4O(AID)6can be used to modificate cathodeof OSC devices efficiently, and theory simulation can be used as a useful tool to explore the mechanism of OSC devices.Compared with spin coating, electrospray (ES) as a new method for OSC devices fabricationhas advantages of controllable process, self-organized nanostructures, low cost and low waste ofmaterials, as well as large-area and flexible devices fabrication. This method has great importancefor future application and industrialization of OSC. ES is used to fabricate the bulk heterojunctionactive layer and the solvent system is optimized. The acetic acid is firstly used as an additive tofabricate P3HT:PCBM active layers by ES. The analysis of active layer morphology and devicecharacteristics reveals that acetic acid is more propriate than other additives. Meanwhile, theconcentration of acetic acid is optimized. The devices fabricated by ES with the optimized solventsystem achieve performance (2.99±0.08%) close to the devices fabricated by spin coating in N2glove box (3.12±0.09%) and significantly higher than devices fabricated by spin coating in air(2.44±0.12%). The comparison of the stability of OSC devices fabricated by ES and spin coatingshows that ES-coated devices are more stable in air.The influence of the process parameters on the morphology of active layer and deviceperformance is investigated systematically. Firstly, the explicit equation of the Damkh ler number,which is related to the evaporation of electrospray droplets, is deduced. Then, the Damkh lernumber during evaporation is adjusted by changing the flow rate and substrate temperature, thus themorphology of active layer is controlled. The appropriate Damkh ler number, as well ascorresponding process parameters, which benefit to obtain better active layer and deviceperformance are demonstrated. Finally, the thickness of active layers fabricated by ES is optimized.Under conditions with the same solvent system and process parameters, OSC devices fabricated byES in air show similar performances (3.09±0.11%) with devices fabricated by spin coating in N2glove box. The results indicate that ES method can be used to replace spin coating in OSC devicefabrication. Meanwhile, the morphology of active layer can be controlled by adjusting theparameters during ES process. Lastly, active layers with complex structures are fabricated by ES.The OSC devices with multi-layer heterojunction achieve efficiency of3.23±0.08%, and OSCdevices with gradient concentration of active materials achieve efficiency of3.39±0.11%. Theresults reveal that ES can be used to prepare active layers with multilayers or gradient materialconcentration conveniently, and these complex active layers are beneficial to charge transport andcollection, and thus to improve the efficiency of OSC devices. In this thesis, the structure optimization, electrode modification and electrospray fabrication ofOSC devices are investigated by combining the theory simulation and device experiments. Someinnovative results are obtained, which lay the foundation of theory and experiment for fabricatingOSC devices with high efficiency, flexiblility and large area.